IPNSHostnameOption() touches the URL path only on the way in,
but not on the way out. This commit makes it complete by
touching the following URLs in responses:

- Heading, file links, back links in directory listings
- Redirecting /foo to /foo/ if there's an index.html link
- Omit Suborigin header

License: MIT
Signed-off-by: Lars Gierth <larsg@systemli.org>